fix assertion bug when mesh does not contain any UV.
fix assertion bug when mesh does not contain any UV.

RealityFactory/ogre,paroj/ogre,RealityFactory/ogre,RealityFactory/ogre,OGRECave/ogre,OGRECave/ogre,paroj/ogre,OGRECave/ogre,RealityFactory/ogre,RealityFactory/ogre,paroj/ogre,OGRECave/ogre,paroj/ogre,paroj/ogre,OGRECave/ogre